In a typical pulverized coal power plant, 80 percent of the ash is carried from the boiler as fly ash and is removed from the flue gas in an electrostatic precipitator, fabric filter or wet scrubber. As a result. U.S. electric utilities spend about $1 billion annually to dispose of most of the 75 million tons of ash removed from their pulverized coal burning plants. For this reason, there has been worldwide activity for many years on the development of ways of utilizing fly ash as an alternative to ash disposal. One of the high volume end uses which has found commercial acceptance is the substitution of fly ash for some of the cement used in concrete.
However, in addition to containing a variety of inert minerals, fly ash may also contain undesirable amounts of unburned carbon. High levels of unburned carbon in fly ash are common with bituminous coals burned in both comer-fired and wall-fired pulverized coal boilers. High levels of unburned coal are even more severe when low NOx burners are used for NOx control. Many utilities who wish to sell their fly ash for use in concrete must reduce these levels of fly ash carbon at high cost. Although over 56 million tons of cement were used to produce concrete in the U.S. in 1991, only approximately 7 million tons of fly ash went into U.S. concrete due to the cost of carbon removal.
One of the standard laboratory tests for the amount of unburned carbon in fly ash is the so-called loss-on-ignition test (LOI). In this disclosure, the terms LOI and unburned carbon will be used interchangeably. The amount of LOI is influenced by the size distribution of coal leaving the coal pulverizers, the combustion conditions in the furnace and the design of he furnace and the burners. For utility boilers burning bituminous coal, utilities have traditionally tried to maintain the LOI in the fly ash to below 6 weight percent. This is done to prevent excessively large losses in boiler efficiency. But for many boilers, this has not been an easy goal to meet. For those utilities who wish to sell their fly ash for use in concrete, it is necessary to maintain the fly ash carbon to even lower levels, with limits of 3 to 5 percent carbon content usually imposed by the concrete manufacturers. Utilities are now installing low NOx burners on their boilers for NOx control. However, as described above, this escalates the problem of fly ash LOI. Test data resulting from recent low NOx installations show that operating a boiler with low NOx burners invariably results in an increase in fly ash LOI, sometimes two-fold in extent.
Thus, there remains a commercial need for an industrial process which beneficiates fly ash in an economical manner, thus making it more amenable to reuse in concrete and otherwise. The problem lies in the separation and removal of carbon from fly ash.
One proposed solution is described in U.S. Pat. No. 5,299,692 issued to Nelson et al. This approach uses an inclined vibrating bed for reducing carbon content. However, this vibratory method was shown to result in 10-80% weight reduction of the carbon content in fly ash. As described above, it is necessary to reduce fly ash carbon to even lower levels on the order of 3 to 5 percent carbon content in accordance with requirements imposed by the concrete manufacturers.
There are analogous methods of particulate separation which have been used for other purposes. For instance, a proven method to achieve separation of pyritic sulfur and ash from the clean portion of coal uses a bubbling fluidized bed operating at room temperature and atmospheric pressure.
FIG. 1 illustrates a bubbling (or gas) fluidized bed 1. At flow rates above minimum fluidization (U.sub.mf), some of the gas flows through the bed from bottom (Z=0) to top (Z=L) in the form of voids of gas or bubbles. As these bubbles move upward through the bed, they cause agitation and motion of the solids, leading to circulation of solid material in the axial direction. When particles of different densities and sizes are contained in the fluidized bed 1, there is a tendency at near minimum bubbling conditions for the solids to stratify in the vertical direction Z according to density, and to a lesser extent, size. When the fluidized bed is used for separating pyritic sulfur and ash from the clean portion of crushed coal, the clean fraction of the coal segregates at the top of the bed, with the liberated coal mineral particles settling towards the bottom. As a consequence, the pyrite content of the coal at the top of the bed (near Z=L) is lowered, thereby permitting recovery of coal with substantially reduced amounts of pyrite. This is shown graphically in FIGS. 2-3, which graphs represent the vertical variation of coal and ash. respectively, after processing in a fluidized bed 1 (as a function of vertical position within the bed).
Specifically, FIG. 2 is a graphical illustration of the local coal concentration (%) as a function of the vertical position Z along the bed 1. It can be seen that the coal concentration increases toward the top of the bed (near Z=L).
FIG. 3 is a graphical illustration of the local ash content of the coal (%) as a function of the vertical position Z along the bed 1. The ash content increases toward the bottom of the bed (near Z=0).
In summary, the clean coal fractions stratify at the top of the bed (Z=L) with the pyrite and ash concentrated toward the bottom (Z=0), and it only remains to separate the clean coal and remove the pyrite and ash concentrations.
An improved continuous process for cleaning coal is disclosed in U.S. Pat. No. 5,197,398. This process, referred to as D-CoP, relies on an inclined fluidized bed (shown in FIG. 4 herein). This type of bed resembles a long, nearly horizontal, table, with fluidizing air passing vertically upward through a distributor causing bubbling to occur in the bed material. The coal and magnetite, which arc added to the bed at one end via coal feed and magnetite feed, flow along the length of the bed, and as they do so, the pyrite and other minerals sink downward through the layer of particles. The clean coal with some magnetite is then separated from the coal refuse at the discharge end. The D-CoP process relies on the use of a bubbling fluidized bed to achieve separation of particles based on differences in density. The bubbles, which are formed at the distributor located at the bottom of the bed, act as pumps. They carry material upward from the bottom of the bed, and at the same time, provide a mechanism by which relatively dense particles near the top of the bed can move downward. Thus, the ability to achieve stable bubbling fluidization is central to the good separation efficiencies which are achieved by D-CoP when used for cleaning coal.
In order to apply this fluidized bed approach to fly ash, it is necessary to achieve good bubbling fluidization. However, fly ash particles are relatively small in size, with mean particle diameters which are typically less than 15 to 20 microns. Particles in this size range do not fluidize well in the bubbling mode. Instead, the particles tend to clump together, causing an unsteady slugging type of fluidization.
In 1994, S. Mori and T. Sawa from Nagoya University in Japan published a paper in Japanese entitled "Development of a Coal Fly Ash Upgrading Process." That paper describes the use of a fluidized bed approach to beneficiate fly ash. However, in the Mori approach, two fluidized beds are used, the first to achieve a size segregation of the fly ash, and the second to process the fine fraction and achieve a density segregation of this material. To deal with the interparticle forces, Mori and Sawa use a mechanically agitated (vibrated) fluidized bed, which is vibrated at a frequency of 25 Hz. The carbon rises towards the top of this mechanically agitated bed and is removed by suction. Mori does not teach the advantages of using an inclined fluidized bed, and the need for mechanical agitatation results in high capital and operating costs.
There remains a need for a viable industrial process to separate materials with particle diameters in this range, and it is herein disclosed how an improved approach for separating, e.g., carbon from fly ash, using an inclined fluidized bed can achieve the desired result.
It is additionally possible to attain even better separation by supplemental acoustic separation.
There has been some previous work on the use of an acoustic field to enhance the fluidization of fine particles. For instance, Chirone et al., Bubbling "Fluidization of a Cohesive Powder in an Acoustic Field," Fluidization VII, Engineering Foundation, (1992), studied bubbling fluidization of a cohesive powder (1 to 45 microns) in an acoustic field. This study reported that the large clusters responsible for the tendency of the powder to spout became converted into fluidizable sub-clusters by the acoustic field, thus making it possible to achieve stable bubbling conditions. Chirone et al. used sound intensity levels in the range of 120 to 150 dB with a frequency of 190 hertz.
Nowak et al., "Fluidization and Heat Transfer of Fine Particles in an Acoustic Field," Fluid-Particles Processes: Fundamentals and Anclications, AIChE Symposium Series No. 296, Volume 89, (1993), found similar beneficial effects of an acoustic field on the fluidization of cohesive powders with sound pressure levels ranging from 100 to 130 DB being required to achieve the desired effect.
To date, there have been no efforts to determine the effect of the acoustic field on those properties of the bubbles which affect solids mixing and segregation.
It would be greatly advantageous to provide an improved industrial process to separate materials with smaller particle diameters such as fly ash using an inclined fluidized bed approach with optional acoustic enhancement. This would allow economical beneficiation of these particulates, thus making them more amenable to reuse (such as fly ash for concrete and other uses).